1. Field of the Invention
The present invention relates to a switching power supply circuit, and more particularly to a switching power source circuit which is effectively applied to, for example, a voltage stabilizing power supply, a DC voltage converter and the like.
2. Description of the Related Art
A switching power supply circuit supplies an input current to a load while switching the input current periodically, and variably controls an ON/OFF time ratio of the switching, whereby the switching power supply circuit controls an output voltage supplied to the load to a predetermined target value. In recent years, this switching power supply circuit has come to be widely used for a local power supply for use in a CPU of a personal computer, a very-small-scale DC/DC converter for use in a portable device and the like.
FIGS. 10A and 10B illustrate an example of a conventional switching power supply circuit. Respectively, FIG. 10A illustrates a circuit diagram of a principal part thereof, and FIG. 10B illustrates a waveform chart of an operation thereof.
The switching power supply circuit illustrated in FIG. 10A is constituted with use of a power MOS transistor Q1 as a switching element; a high frequency power transformer 10; diodes D1 and D2; a choke coil L3; a voltage comparison circuit 21; an LPF (low pass filter) 22; a PWM (pulse width modulation) circuit 23; a clock generator 25; and the like.
In the above described circuit, the transistor Q1 is interposed in series between an input power supply of a voltage Vi and a primary coil L1 of the transformer 10, and switches an input current flowing into the primary coil L1. Thus, a switching output current appearing at a secondary coil L2 of the transformer 10 is rectified by the diode D1, and thereafter is supplied to a load via the choke coil L3. The load has a capacitance Cx and a resistance Rx equivalently, and forms a time constant of a smoothing circuit together with the choke coil L3.
An output voltage Vo supplied to the load is compared with a predetermined reference voltage Vr by the voltage comparison circuit 21. Since ripples are contained in a comparison output voltage from the comparison circuit 21, the comparison output voltage is converted to a DC voltage by the LPF 22. The LPF 22 averages the comparison output voltage on a time axis so that the comparison output voltage has a sufficiently large time constant relative to a cycle of the ripple, that is, a switching cycle of the transistor Q1.
The comparison output voltage which has been converted to the DC voltage by the LPF 22 is supplied to the PWM circuit 23 as a pulse width control signal Vm. The PWM circuit 23 operates in synchronization with clocks φ, and generates pulse signals Vp having a constant cycle T to allow the transistor Q1 to perform a switching operation. A pulse width of the pulse signals Vp, that is, an ON/OFF time ratio (t1/t2) of the switching is feedback-controlled by the pulse width control signal Vm.
In other words, the foregoing switching power supply circuit detects a difference between the output voltage Vo as a control value and the reference voltage Vr as a target value, that is, a time average value of a control error, and converges the output voltage Vo to the reference voltage Vr by a feedback control loop which makes the time average value zero. The foregoing switching power supply circuit presents no problem especially if a change in the load is comparatively slow. When a load current changes rapidly and greatly like, for example, a local power supply for use in a CPU of a personal computer, the foregoing feedback control loop can not sufficiently follow the change in the load, and a so-called response delay and an excessive response occur. As means for compensating an inadequacy in this response characteristic, a contrivance in which a large-capacity smoothing capacitor (capacitor element) is inserted in parallel with an output line is adopted frequently. However, in order to adopt such contrivance, a large-scale capacitor having a capacity large enough to fully absorb the change in the load current is required, and this contrivance is not fundamental problem-solving means. For example, though a recent high-speed CPU is designed to consume a large current with a low voltage, there is a problem in that the smoothing capacitor alone is allowed to absorb a current change in a low impedance load as described above. On the other hand, when a response speed is increased to cope with the rapid change of the load in the foregoing conventional switching power supply circuit, a problem occurs in which a loop operation of the feedback control becomes unstable and troubles such as oscillation are apt to occur.